Avalanche photodetectors (APD) provide higher sensitivity than p-i-n photodetectors because of the internal gain from avalanche multiplication. APDs are useful in optical receivers for a number of applications. Due to the uncertainties of the total number of impact ionizations, there is amplitude noise on the avalanche gain. The noise is determined by avalanche multiplication material, characterized by the ionization rate ratio between electrons and holes, k. Similar ionization rate between electrons and holes, i.e., k≈1 corresponds to high noise, while low ionization rate between electrons and holes, i.e., k≈0 corresponds to low noise.
Silicon is transparent to the a set wavelengths used in optical fiber communication systems, 1.3 μm-1.6 μm, so epitaxial germanium is typically used for light absorbing material in photodetectors in silicon photonics. However, germanium has a k close to 1, making it a noisy avalanche material. On the other hand, silicon has a very small k<0.1, which is preferable for avalanche. Thus, prior art APDs usually have separate absorption and multiplication regions, as shown in FIG. 1.
The electric field in different layers is illustrated in FIG. 2. In the absorption region, the electric field needs to be high enough to drive the photo-generated carriers at their drift saturation velocity, while low enough to avoid avalanche multiplication, which sets it in the 10 to 100 kV/cm range. In the avalanche region, the field needs to be high enough, greater than 300 kV/cm, for efficient multiplication.
The conventional prior art APD has a complicated layer structure, which requires multiple epitaxy and doping steps. Typically this type of geometry would be used for vertical incidence detection where light is traveling perpendicular to the plane of the chip. However, integrated optics require waveguide-coupled detectors in which the light is travelling in the plane of the chip. It is difficult to convert the conventional APD structure to work as a waveguide coupled device due to its numerous epitaxial steps.
There is a need for improved avalanche photodiode device structures that allow simpler and less costly fabrication.